The present invention relates to an offset blanket and rubber rolls used for offset printing.
In printed matter such as filter or display prints or the like in which ink layers printed on the surface of a transparent member are seen through transmitted light, thickness variations in the ink layers, if any, are represented in terms of difference in color shade. It is therefore required that the ink layers have a substantially even thickness.
In offset printing using an offset blanket, however, a major portion of the ink transferred from the printing block to the blanket surface is not transferred to the surface of the member to be printed on, but remains on the blanket surface. Accordingly, the ink layers on the surface of the member to be printed on disadvantageously present concavo-convex portions due to cohesive failure, causing the layer thicknesses to become considerably uneven. Further, the print edges cannot be clearly reproduced.
A normal printing offset blanket has a supporting layer which incorporates or does not incorporate a porous compressive layer, and a surface printing layer on the supporting layer. The surface printing layer is made of a highly oil-resisting rubber material, mainly an acrylonitrile-butadiene copolymer (hereinafter referred to as NBR).
However, when printing is conducted at a high speed with the use of such a normal offset blanket, an adhering force is produced between the paper and the blanket. This may cause the paper to be curled or broken. Similar problems are encountered at the time of printing on a smooth member such as coated paper or the like. These problems are generally encountered when so-called paper discharging properties (paper releasing properties) are poor. Since such troubles greatly lower the productivity, the blanket is required to present good paper discharging properties.
To improve the paper discharging properties of a conventional offset blanket, there have been proposed a variety of methods such as a method in which polishing the surface of the surface printing layer is so conducted as to make the polished surface coarse, a method in which starch incorporated in the surface printing layer is first vulcanized and then extracted by being dissolved in a solvent (Japanese Patent Publication No. 238/1991), a method in which ultraviolet rays are irradiated onto the surface of the surface printing layer (Japanese Patent Unexamined Application No. 37706/1976), a method of surface chlorination (Japanese Patent Publication No. 51729/1972) and the like.
However, according to the method of making the surface roughness coarse or forming small holes in the surface, the contact area of the printing layer with paper is reduced to deteriorate the net-point shape, thus reducing the reproducibility of the net points. According to the method of surface chlorination, micro-cracks are produced in the surface of the surface printing layer, resulting in deterioration of net-point reproducibility, washing quality and the like. The method of surface treatment with ultraviolet rays is effective in improvements in paper discharging properties and is excellent in net-point reproducibility. However, this method requires not only an ultraviolet irradiation installation but also strict control of irradiation dose.
In addition to paper releasing properties, attention should be given to the problem of paper powder accumulated on the offset blanket due to long-term printing. More specifically, regenerated paper is increasingly used with the trend of resource conservation and recycling. However, the regenerated paper, which is deteriorated in quality, is liable to produce paper powder. Accumulation of paper powder on the off-set blanket provokes problems of decrease in printing quality, increase in the number of washing steps, and the like. To lessen the generation of paper powder, it is effective to make the surface roughness of the surface printing layer coarse as was done for improvement in paper discharging properties. It is however difficult to reduce the retention of paper powder without injuring the net-point reproducibility.
On the other hand, recent offset printing is liable to use ultraviolet-curing ink (UV ink) in order to prevent the working environment from being polluted by solvent components evaporated from ink, as well as for more efficient printing at a higher speed. With the diversification of a member to be printed on (particularly, plastics), the demand for the ultraviolet-curing ink is increased, particularly in the field of food packing, which should be kept free from printing smell.
In a commercially available offset printing machine, as shown in FIG. 13, ink transfer rolls 5a, ink kneading rolls 5b and ink applying rolls 5c are disposed from an ink reservoir 6 to a printing cylinder 7 for kneadingly supplying ink 8 to the printing cylinder 7. In view of the strength and affinity with oil ink, such rubber rolls 5a, 5b, 5c are generally made of a material mainly, that is at least more than about half, made of oil-resisting rubber such as NBR, urethane rubber or the like, or plastic such as polyvinyl chloride or the like. However, when ultraviolet-curing ink is used in an offset printing machine using such oil-resisting rubber, the rolls are disadvantageously swollen or decreased in outer diameter due to elution of additives, such as plasticizer or the like, contained in the rubber rolls. As a result, the rubber rolls cannot be used for a long period of time, requiring frequent replacement.